1. Field of the Invention
The present invention relates to a staged feeding process for mixing at least two polymers to prepare a composition comprising a uniform, small-particle dispersion of one polymer in a matrix of another polymer, where the one polymer has a melt viscosity greater than the other polymer. In particular, the present invention relates to a process for mixing rubber and nylon to obtain a fine dispersion of rubber in nylon, where the viscosity of the rubber greatly exceeds that of nylon.
2. Description of the Related Art
Toughened engineering polymers, sold under the trademarks Zytel.RTM., Delrin.RTM., Rynite.RTM., Minlon.RTM., and Bexloy.RTM., all registered trademarks of E. I. du Pont de Nemours and Company of Wilmington, Del., have mechanical properties that meet a wide range of automotive, appliance and construction industry strength and longevity requirements. To enhance toughness and impact strength of these materials, fine rubber droplets may be dispersed in the polymer matrix of these materials. The impact strength of these materials is improved dramatically when the droplets are below a desired size.
Processes for mixing dissimilar, or immiscible polymers, i.e., polymers having non-zero interfacial tensions, as solids in a single stage are known. For instance, U.S. Pat. No. 4,174,358 to Epstein discloses the preparation of toughened thermoplastic polymers by the dry blending of the solid constituents with subsequent coincidental mixing and melting. A fine dispersion of rubber in a polymer matrix is formed by feeding pre-blended pellets of polymer and rubber to a co-rotating twin-screw compounding extruder. In this process, the extruder melts the solid pellets and simultaneously mixes the polymer and the rubber. The rubber is well dispersed as droplets of the desired size in the final composition.
Attempts have been made to introduce all of the nylon and rubber as melts simultaneously into a mixer in a single-stage feeding process in amounts that will produce a preponderance of the nylon. The nylon and the rubber are mixed in a flow field of rotational shear. However, this process fails to produce a fine dispersion. Because of the preponderance of nylon in this all-melt process, the nylon constitutes the continuous phase. The far more viscous discontinuous phase (rubber and functionalized rubber) does not break into small droplets. This is not surprising since it has been found that a fine dispersion of a more viscous polymer in another, less viscous polymer will not occur if the viscosity of the more viscous polymer is more than about 3.5 times that of the less viscous polymer, as explained by H. P. Grace's experiments with immiscible, Newtonian fluids. See H. P. Grace, Dispersion Phenomena in High Viscosity Immiscible Fluid Systems and Application of Static Mixers as Dispersion Devices in Such Systems, published in Chemical Engineering Communication, Vol. 14, 225-277 (1982).
On the other hand, when the known solid-feed process as described above is performed with rubber and nylon, it creates a more viscous continuous phase of rubber. As the nylon melts, it forms a discontinuous phase of less viscous droplets which break readily into smaller droplets because of the viscosity ratio of the discontinuous phase to the continuous phase. This enables grafting to occur, which leads to further dispersion due to a reduction of interfacial surface tension between the rubber and nylon.
When the volume fraction of two polymers is roughly equivalent, upon coincidental melting and mixing in a single stage process, they combine to form a morphological state described as co-continuous, or having a dual-phase continuity, in which one cannot identify a distinctive, discontinuous fluid phase. Thus, no dispersion is created by this mixing. This result is described by B. D. Favis and J. P. Chalifoux in an article titled "Influence of Composition on the Morphology of Polypropylene/Polycarbonate Blends" in Polymer, Vol. 29, pp. 1761-1767 (October, 1988), which discusses blending polypropylene and polycarbonate in a batch mixer.
Staged feeding processes for mixing similar, or miscible, polymers to obtain a homogeneous, multi-component, single-phase composition are also known. For instance, Published Patent Specification DT 2 551 352 to Zettler et al. discloses a staged feeding process for mixing similar components where a low-viscosity component is added to a high-viscosity component already present in an extruder. In a specific embodiment, a high-viscosity component (a high-molecular weight polyisobutylene component) is fed as a solid to a first stage of an extruder, and a low-viscosity component (a low-molecular weight polyisobutylene component) is fed as a melt to subsequent stages of the extruder. The result is a gel-free, homogeneous, single-phase composition consisting of 6% of the high-molecular weight polyisobutylene component and 94% of the low-molecular weight polyisobutylene component.
It is also a known process to feed all rubber and part nylon as solids to a first stage, and to subsequently feed the nylon as melt to latter stages. However, the rate of manufacture of this process could be improved.